Precipitation Titration Mohr Method

1 January 2017

The chloride ion (Cl-) is an important anion found in solids and solutions. In this experiment, the amount of chloride ion in an unknown sample J (NaCl + KCl mixture) of water using the Mohr method is determined, which relies on the solubility differences of two anions and the titration endpoint of a precipitate. The net ionic reaction during the titration is as follows: Ag+ + Cl- > AgCl(s) The Ksp for AgCl is 1. 8 x 10-10 and that for Ag2CrO4 is 1. 2 x 10-12.

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Thus, as silver ions are slowly added to solution, AgCl precipitates first, removing the chloride ions from solution. As can be inferred, Ag+ ions react with almost all chloride ions before precipitation of Ag2CrO4 begins. Thus, the secondary reaction is 2Ag+(aq) + CrO42-(aq) > Ag2CrO4(s) and results in a color change from yellow to reddish-brown at the end point. Knowing the molarity of the titrant allows for calculation of the chloride concentration. Materials: volumetric pipettes, tubes, 250 mL Erlenmeyer flasks, 50 mL buret, 16. g/1L of AgNO3, CrO4 solution.

Weigh out the sample (NaCl + KCl mixture) into a dry tube in a range of 0. 150 g to 0. 250 g. Dissolve the salt with water to 10 mL/15 mL/20 mL. Pipette a 5 mL aliquot of diluted salt into a 250 mL flask and add distilled to 100 mL mark. Then, add 1 mL of CrO4 indicator to the solution. Titrate the sample with silver nitrate solution. The endpoint of the titration is identified as the first appearance re-brown color of silver chromate. Important details: Silver nitrate is obtainable in primary standard purity.

It has a high equivalent weight and dissolves readily in water. The solid as well as its solutions must be scrupulously protected from organic matter and from sunlight. The reagent is expensive; every effort should be made to avoid waste. Unused solutions should be collected rather than discarded; similarly, appreciable amounts of silver chloride should be collected. Mohr titrations should be performed at room temperature.

Data/Calculation Molecular mass AgNO3 = 169. 87 g/mol Calculation for tube 2 of unknown: Molarity of AgNO3 = (16. 7 g)/L? (1 mol)/(169. 7 g)= 0. 098 M AgNO3 0. 098 M AgNO3 ? 0. 0101 L = 0. 0009898 mol AgNO3 mol AgNO3 = mol Cl- at equilibrium point 0. 0009898 mol AgNO3 = 0. 0009898 mol Cl- Titrated solution: 100 mL Dilution Factor: 0. 05 (0. 0009898 mol )/((0. 1L)(0. 05)) = 0. 19796 M of Cl- (0. 19796 mol Cl)/L? (0. 015 L)/? (35. 45 g)/(1 mol Cl) = 0. 1052 g of Cl- (0. 1052 g Cl)/(0. 17 g of sample J)? 100% = 61. 92% of Cl- in sample J % Cl- in sample J = 52. 73 ± 0. 95 Table 1. Determination of Chlorine in original sample TubeGrams of sample JVolume (mL) of AgNO3 titrated % Cl- in original ample 20. 17010. 161. 92 30. 2109. 044. 67 40. 21010. 351. 12 50. 1909. 753. 21

Table 1 depicts the various volumes of titrant used to titrate sample J. Table 1 also shows the percent of Chlorine in the original sample. The percent of Chlorine varies based on the weight of the original sample and the amount of titrant used. Discussion The well known Mohr’s method in which alkaline or alkaline earth chlorides react with silver nitrate in the presence of 1 mL of Chromate as indicator is a simple and direct method for chloride determination.

Since the solubility of silver chloride and silver chromate depend on temperature, all titrations were carried out at about the same temperature. Good stirring during the addition of the silver nitrate is also required for reproducible end point to ensure a homogenous mixture. Based on the color of the solution after titrating, it can be hypothesized that the equivalent point may have been surpassed due to the red color of the solution. My results are reasonable because of the percent of chlorine calculated are roughly 50% of the original sample.

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